Communication and Networks Assignment
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0_Lesson16PacketandCircuitSwitching.pptxCommunications and Networks
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Diploma in Information Technology
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Lesson 16: Packet and Circuit Switching
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Lesson 16 Learning Outcomes
Distinguish circuit and packet switching
Explain the IEEE 802 model and standard
Describe the concept of unicast, broadcast and multicast
Describe the delivery of broadcast and multicast
Explain the purpose of bit and byte stuffing
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Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing
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Multi-access Network
Multi-access Network: multiple computers to share a medium in such a way that any computer on the LAN can communicate with any other
In general, LAN technologies provide direct connection among communicating entities
However, professionals say that LANs connect computers with the understanding that a device such as a printer can also connect to a multi-access LAN
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LAN Topologies
Each network is classified into a category according to its topology or general shape
Bus Topology
Ring Topology
Mesh Topology
Star Topology
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Bus Topology
Bus topology consists of a single cable
Ends of bus network must be terminated to prevent electrical signals from reflecting back
Computers must coordinate to ensure only one computer sends a signal at any time
Source: Douglas, C (2016) Computer Networks and Internets
| Advantage | Disadvantage |
| All nodes share access to a common medium | Not resilient to failures |
| Easy to implement | Single cable fault will split the network in two |
| Adding new node is easy | Isolating a fault is difficult |
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Ring Topology
Ring topology: computers are connected closed loop
In practice, cables can run along hallways or rise vertically from one floor of a building to another
| Advantage | Disadvantage |
| Resilience to single failure provided data travel around the ring in either direction | Harder to implement |
| Faults are easier to isolate | Use more cable |
| Adding new node is difficult |
Source: Douglas, C (2016) Computer Networks and Internets
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Mesh Topology
Mesh topology: provides a direct connection between each pair of computers
Costly to connect n computers:
| Advantage | Disadvantage |
| Relatively easy to manage | Expensive if there are large number of nodes |
| Relatively easy fault isolation | Adding new nodes is difficult and expensive |
| Extremely resilient to multiple failures |
Source: Douglas, C (2016) Computer Networks and Internets
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Star Topology
Star topology: all computers attach to a central point often called a hub
Hub is typically a network device
Source: Douglas, C (2016) Computer Networks and Internets
| Advantage | Disadvantage |
| Relatively easy to add new nodes | Not particularly resilient to failures especially at hub or central site |
| Faults are easier to isolate | |
| Easy to manage as network equipment is centralized |
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Practice 16.1
For each of the following, determine a suitable network topology to use:
Resilient against single point of failure
Easy fault isolation
Cost effective
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Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing
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Packet and Circuit Switching
Source: https://www.youtube.com/watch?v=gB0DCb84T7c
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Circuit Switching
Circuit switching: a communication mechanism that establishes a dedicated path between sender and receiver when needed
Guarantees isolation from paths used by other pairs of senders and receivers
Usually associated with telephone technology as it provides dedicated connection between telephones
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Virtual Circuit
Circuit switching networks use electronic devices to establish circuits
Instead of a physical path, multiple circuits are multiplexed over shared media
Resulting to a virtual circuit
Source: Douglas, C (2016) Computer Networks and Internets
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Properties of Circuit Switching
Point-to-point communication as circuit is formed between exactly two endpoints
Separate steps for circuit creation, use, and termination, which distinguishes switched vs permanent circuit
Performance equivalent to isolated physical path
Communication between two parties is not affected by others
Provide illusion of an isolated path for each pair of communicating entities
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Steps of Circuit Switching
Three-step process analogous to placing a phone call
Establish a circuit between two parties
Two parties communicate using the circuit
Two parties terminate use of the circuit
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Packet Switching
Packet switching system uses statistical multiplexing
Multiple sources compete for a shared media
Source: Douglas, C (2016) Computer Networks and Internets
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Packets
Packet switching system requires a sender to divide each message into smaller blocks of data
Packets: blocks of data
Size of a packet can vary
Each packet switching technology defines a maximum packet size
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Properties of Packet Switching (1/2)
Asynchronous communication: allows sender with one or more recipients and recipient with one or more senders at any time
No set-up required: can deliver a packet to any destination at any time and no need for initialisation before communicating
No need to notify underlying system when communication terminates
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Properties of Packet Switching (2/2)
Performance varies due to statistical multiplexing among packets
Multiplexing occurs among packets rather than among bits or bytes
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Circuit vs Packet Switching
Packet switching incurs lower cost from sharing
To provide communication among N computers:
Circuit-switched network must have a connection for each computer plus at least N/2 independent paths
Packet switching network must have a connection for each computer but only requires one path that is shared
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Packet Switching Technologies
Least expensive networks use technologies that span a short distance like inside a single building
Most expensive span long distances like across several cities
Source: Douglas, C (2016) Computer Networks and Internets
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Packet Switching Identification
Each packet sent must contain identification of intended recipient
Senders agree on exact details of how to identify recipient and identification field in a packet
Standard organisations specify these details
IEEE organised Project 802 LAN/MAN Standards Committee to produce standards for networking
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Practice 16.2
What are the steps to setup, communicate and terminate circuit switching?
Which property of packet switching allows multiple pairs communication to take place? Explain.
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Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing
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Standard Organisation Focus
IEEE is mostly focus on the lower two layers of the protocol
Other standard organisations focus on other layers of the stack
Source: Douglas, C (2016) Computer Networks and Internets
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IEEE Data Link Specification
IEEE divides Data Link into two conceptual sublayers
Logical Link Control (LLC): specifies addressing and the use of addresses for demultiplexing
Media Access Control (MAC): specifies how multiple computers share underlying medium
Source: Douglas, C (2016) Computer Networks and Internets
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IEEE Standards
IEEE assigns multi-part identifier of the form XXX.YYY.ZZZ
XXX: category of standard
YYY: subcategory
ZZZ: reserved for 3rd category
Source: Douglas, C (2016) Computer Networks and Internets
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IEEE Working Groups
IEEE created many working groups, each intended to standardise one type of network technology
Working group consists of representatives from industrial and academic communities
IEEE allows working group to be active provided the group makes progress and technology is still deemed important
If a working group decides that a technology is no longer relevant, it can decide to disband
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Packet Addressing
IEEE has created a standard known as Media Access Control (MAC) address or Ethernet address, each consisting of 48 bits
Each packet that travels across the shared medium contains MAC address of specific recipient
Source: Douglas, C (2016) Computer Networks and Internets
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Demultiplexing Packets
Demultiplexing of packets make use of the MAC address
IEEE allocates a unique address for each piece of interface
Each Network Interface Card (NIC) contains a unique address assigned when the device was manufactured
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Assigning MAC Address
MAC Address consist of:
3-byte Organisationally Unique ID (OUI) to identify the equipment vendor
3-byte unique values assigned by vendor that identifies a NIC
Source: Douglas, C (2016) Computer Networks and Internets
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Address Type
IEEE addressing supports three types of addresses that correspond to different types of delivery
Source: Douglas, C (2016) Computer Networks and Internets
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Unicast, Broadcast & Multicast
IEEE address reserves a bit to distinguish between unicast and multicast but does not provide a way to designate a broadcast address
Broadcast address consists of all 1s
Broadcast is a special form of multicast
Multicast is for specific group of computers
Broadcast includes all computers on the network
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Broadcast & Multicast Efficiency
In a typical LAN:
Each computer monitors the shared medium
Extracts a copy of each packet
Examines the address in the packet
Determine whether the packet should be processed or ignored
Broadcast and multicast are useful in LANs as they permit efficient delivery to many computers
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Practice 16.3
What are the TWO (2) components in a MAC address?
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Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing
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Framing
Framing is the the structure added to a sequence of bits or bytes that allows sender and receiver to agree on exact format of the message
In a packet-switched network, each frame corresponds to a packet that consists of two conceptual parts:
Header: data like address and information used to process the frame
Payload: data being sent
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Frames
A message is opaque: network only examines the frame header
Payload can contain bytes that are only meaningful to the sender and receiver
Some technologies represent frames by sending a short prelude before frame and short postlude after
Source: Douglas, C (2016) Computer Networks and Internets
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Frame Example
Assume a packet header is 6 bytes and the payload consists of an arbitrary number of bytes
Can use ASCII characters where
Start Of Header (SOH) character for the beginning
End Of Transmission (EOT) character for the end
Source: Douglas, C (2016) Computer Networks and Internets
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SOH and EOT Characters
In the ASCII character set:
SOH hexadecimal value is 201
EOT hexadecimal value is 204
If the payload of a frame includes value 201 or 204, can use byte stuffing to allows transmission of without confusion
SOH and EOT cannot appear in the payload
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Bit and Byte Stuffing
Frame delimiters can be used to distinguish between data and control information
Sender changes a data to replace each control byte with a sequence
Receiver replaces sequence with the original
This is known as byte/data/character stuffing
Bit stuffing: a related technique used with systems that transfer a bit stream
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Byte Stuffing Example
Byte stuffing reserves a third character to mark occurrences of reserved characters in the data
Sender can replace the special characters with a two-character sequence
An example is using the ESC character:
Source: Douglas, C (2016) Computer Networks and Internets
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Byte Stuffing on Receiver
Receiver reverses the mapping
Looks for ESC followed by one of A, B, or C
Replace the 2-character combination with either SOT, EOT or ESC
Once byte stuffing has been performed, SOH EOT and ESC will not appear anywhere in the payload during transmission
Only appear after receiver reverse the mapping
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Byte Stuffing Illustration
(a) is the original message
(b) is the message after byte stuffing
Source: Douglas, C (2016) Computer Networks and Internets
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 13
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End of Lesson
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